Improve flathub automated submission process

[Blooym]

Right now every time we tag a release it makes a pull request to the flathub repository - we've been bitten by this more times than it's helped us save time so it's best we change it.

Some options:

• A manually triggered workflow that makes a pull request with the last tag/release found on the repository
• Uploading all the required files to the release workflow as artifacts and having a maintainer submit those manually afterwards (could generate a .patch file?)

Open to suggestions on improving this process